Electrical power transformers, which are for example used for energy generation, energy transfer, energy distribution or for industrial applications and are designed for outputs greater than 1 MW, often comprise so-called tap changers in order to be able to adapt the transmission ratio of the respective transformer, for example to certain load conditions. Tap changers, which can also be switched during the operation of the transformer or under load, i.e. without interrupting the operation, are often also designated as on-load tap changers (OLTC). To this end, the winding of the transformer is designed as a so-called bank winding, wherein taps or tappings of this bank winding are guided to the tap changer which then switches over from one tap to another tap during the operation of the transformer in order to thereby change the transmission ratio of the transformer.
Since the windings of a transformer respectively constitute a large inductivity, a switching process between different transmission ratios cannot be implemented without problems since it is not possible to easily interrupt the current flow through a winding. A tap changer is thus designed such that the current can flow through the winding at any time. To this end, the tap changer often switches over the current, which flows through the winding, by means of resistors in order to thereby change the transmission ratio. The switching process must be performed quickly in order to avoid excessive heating of the resistors. Regular testing of each tap changer is recommended due to the complex and thus error-prone mechanics of the tap changers.